Keyed automatic gain control and synchronizing signal separator



B. S. VILKOMERSON ETAL KEYED AUTOMATIC GAIN CONTROL AND SYNCHRONIZING SIGNAL SEPARATOR VFiled Oct. 29, 1949 March 24, 1953 zal /4 fa f5 Patented Mar. 24, 1953 KEYED AUTOMATIC GAIN CONTROL AND SY NCHRONIZING SIGNAL SEPARATOR Benjamin S. Vilkomerson, Camden, and Clyde W. Hoyt, Pennsauken Township, Camden County, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application October 29, 1949, Serial No. 124,416

(Cl. FX8-7.3)

12 Claims.

The present invention relates to an improved form of combination automatic gain control circuit and amplitude discriminatory circuit for radio receiving equipment.

With more particularity, although not necessarily exclusively, the present invention deals with an improved form of electrical circuit for achievingr automatic gain control and sync clipping in present day television receiving arrangements. Even more directly, the present invention is involved with a combination keyed automatic gain control circuit and sync clipping circuit for use in television receivers.

In the radio communication art and in particular, that phase of the art dealing with the transmission and reception of television signal information, it is quite well known that it becomes desirable to supply the receiving equipment with some form of automatic gain control circuit whereby the brilliance and contrast of the reproduced picture is rendered less dependent upon received signal strength. Furthermore, in all television receiving circuits adapted to receive and utilize the present standard composite television signal, it is necessary to provide some form of amplitude discriminatory circuit which will separate relatively high amplitude sync pulses of the composite signal from the remaining lower amplitude portions of the signal. After having been clipped from the composite signal, the sync signals are then applied to the television receiver deflection circuits for timing thereof.

In order to achieve the above-mentioned circuit actions, namely automatic gain control and signal clipping in television receivers, numerous circuit arrangements have been proposed. For instance, in the case of automatic gain control arrangements, circuits are quite well known for producing a gain control potential in accordance with peak detection of the incoming signal, which type of system is relatively vulnerable to noise bursts and rapid signal fading or iiutter. To overcome these disadvantages, automatic gain control circuits of the keyed variety were developed wherein the incoming signal was sampled only during predetermined time intervals, usually corresponding to the composite signal synchronizing level or blanking level. Since the keyed type of automatic gain control potential is developed only in accordance with information sampled from the incoming signal at these predetermined intervals, the immunity of the control potential to'noise bursts is greatly increased.

,Unfortunately however, the keyed type automatic gain control circuit complexities represent additional circuit cost.

Furthermore, in the attempt to combine a plurality of circuit functions into circuits requiring but a few tubes it has generally been found possible to produce combination type automatic gain control and sync clipping circuits, the gain control circuit not,.however, being of the keyed variety. Thus, one of the considerations in television circuit organization becomes the provision of some form of keyed automatic gain control circuit which will additionally provide suitable amplitude discrimination vfor purposes of sync clipping.

Moreover, in all types of sync clipping circuits, it is desirable that the clipped sync information have as sharp and steep a waveform as possible, thereby to enhance the accuracy of deflection circuit timing.

It is, therefore,.the purpose of the present invention to provide an improved form of combination automatic gain control circuit and amplitude discriminatory circuit for radio receiving equipment. p

It is another object of the present invention to provide a new or improved form of keyed automatic gain control circuit which also provides sync clipping action in present day television receiving circuits.

A still further objectof the present invention resides in the provisionof a sync clipping circuit for television receivers which produces exceptionally steep waveform sync separation.

It is further well known that some automatic gain control circuits for radio receivers generally exhibit some time delay in their action. That is to say, some nominal time period is required for the circuit to fully act in compensating for a change in received signal level. This automatic gain control delay is particularly disadvantageous in television receiver circuits using a simple gate type arrangement for sync clipping. With such simple gate circuits, a momentary increase in signal strength may cause the gate circuit to accept and pass video information well below the sync pedestal level and thereby cause mis-synchronization of the deflection circuits.

It is therefore another object of the present invention to provide a new and improved sync clipping circuit for television receivers which automatically immunizes itself against the pass ing of substantial signal information upon the receipt of large amplitude radio carriers.

. In the realization of the above objects and advantages, the present invention in its more general form contemplates the use of a multicontrol electrode electron tube having substantially no static polarizing potential applied to its anode but having moderate polarizing potential applied to its screen electrode. Bias is then applied to a control electrode to establish static cutoff of the screen cuirent. Applied video signals having sync positive polarity may then be applied to a control electrode to establish screen current conduction only during sync peaks thereby allowing separated sync to appear at the screen electrode of the tube. Positive going timing pulses are then applied to the'anode of the electron tube, the timing pulses being, in turn, synchronized with the received sync pulses to produce anode-cathode current during reception of sync. By placing a suitable time constant in the anode circuit, an automatic gain control potential may then be developed which is of a negative polarity and suitable for direct application to the control electrodes of vacuum tube amplifiers handling incoming video signals.

'Other objects and advantages of the present invention in addition to those set forth hereinabove, as well as a more complete understanding of its operating mode, will become apparent to those skilled in the artupon perusal of the following specication, especially when taken in connection with the accompanying drawing.

Turning now to Figure 1, there is indicated in block IG some of the typical components of a present day television receiver, namely, an R. F. tuner, a sound channel, a video I. F. channel and a video detector. Accordingly, signals intercepted by the antenna i2 are amplified, heterodyned and demodulated by the video detector and applied to some form of video ampli- Iier, such as indicated in block le. For purposes of illustration, we shall assume the signal appearing at the output of D.C. amplifier ifi is polarized in the sync positive direction as indicated by the waveform i8. This permits the video signal IS to be directly applied to the cathode i8 of the cathode ray kinescope 2Q for modulation of the electron beam thereof. Any

. form ofv brightness control may be used in the connection with the kinescope 29,l thatv indicated by the voltage source 22 and potentiometer 2Q in connection with the grid 2t being only exemplary. The cathode of kinescope i8 is seen to be further connected through resistance 2l to some positive bias source terminal 29. rihis is indicated only for purposes of" illustrating the necessary D.C. consideration which will hereinafter be entertained. As is quite common in television receiver design, the R. F. and I. F. stages of block I are provided with automatic gain control potential terminals, respectively indicated at 23 and 3i). It is to these terminals that automatic gain control potential may be applied regardless of its mode of development, for adjusting the gain of the R. F. channels and video I. F. channels.

The automatic gain control circuit for developing the gain control potential for application to the terminals 28 and Sii, in accordance with the present invention, comprises a vacuum tube 32 having an anode Bil, a cathode 3S and a plurality of control electrodes. By way of example, the vacuum tube 3,2 has been illustrated as being of the pentagrid converter type, of which the commercial type 6BE6 istquite representative. The positive going video signal I6 is then applied through resistor 38 to the signal grid 4G of the vacuum tube 32. The screen grids i2 and t3 are connected together and thence to a positive biasing potential terminal lr6 through resistors 48 and E Capacitor 52 acts conjunction with resistor t8 to form a special type of compensating circuit whose purpose is to be hereinafter described. The insertion grid 5e of the converter type tube 32 is shown connected with the anode te.

in further accordance with the present invention, cathode 36 of the vacuum tube .'52` may be adjusted in potential by means of the potentiometer 55 connected between the power supply potential available between terminal 58 and ground. Any resistance of the potentiometer 56 inserted in the lcathode circuit of the vacuum 32 is suitably bypassed by the capacitor 68. In order to establishperiodic conduction in the vacuum'tube 32 iorpurposes of automatic gain controlsampling, positive going pulses G2 are extracted from the anode t!! of the deflection output tube 65. As well known by those skilled in the art, these pulses 52 correspond to the iirst half cycle of free resonance in the transformer t8 during the return time, of the4 deection cycle. The pulses 62 are coupledthrough Capacitor le and resistor 'i2 to the anode .54. This capacitorresistor combination both attenuates the amplitude of the pulses so appliedl t'o the anode and blocks the D.C. component appearing at the deflection output tube anode. Forv reasons hereinafter described, the timing or keying pulses 62 are integrated by means of capacitor 'iii in their application to the anode $4. The automatic gain control potential thereby developed at the anode Se is suitably filtered by means of resistance 1G and capacitor 'E8 for application to the Il.r l?. automatic gain control potential Se` while being isiltered by resistance 8,0 and capacitor 52,- for application to the` I. F. automatic. gain Control '0.61'.- minal 2,8. A diode gli acts to prevent the R. F. automatic gain control terminal 28 from going positive Awith respect to ground during rio-signal conditions, the termi-nal 28 being, as shown, connected to a source of positive potential it through resistor 8 8.

In the Operation of the embodiment 0f Fsure 1 according to the present invention, the stati-c grid cathode` bias on the vacuum tube 3.2 is adjusted by means of the potentiometerv 56 and the choice of value of resistance 2j' to establish current cutoff inthe screen electrodes t2 and i3 and thereby through the resistance. elements i8 and 5S. The valuel of the cutoitbias sov applied is. further adjusted to permit electron conduction through the load resistors 48, andeonly for signal amplitudes in excess of the blanking level its of the video signal it. This will then'allow the sync pulses ib to appeal'l in a negative going direction across the screen load resistor 48 for application to the control electrode. of the sync ampliertube 9B. TheV anode load impedance` 9 2 of the sync amplifier tube 9G will then evidence positive going sync pulsesl lb across its terminals which pulses are suitable for conventional synchronization of the horizontal and vertical deiiection circuits 94 and 9 6, respectively. The horizontal 4sync pulsesl appearing at the output of the sync amplifiers Si! will, of course, be differentiated by means: ofl capacitors and resistor lilo, while theverticalsynchronizing pulses will be conventionally integrated bythe integrating network |62, its, H16; Hi8, Illl and H2.

The flyback or timinglpulsesli applied to the anode Sil of the. vacuum tube 32. will then bein synchronism with the incoming horizontal synchronizing pulses and, if` properly adjusted in amplitude by proportioning resistor 12 relation to other circuit impedances, the. anode 3.4, of the vacuum tube can be made'. to, swing sufficiently positive to produce anode-cathode current iiow during the reception of sync pulses. It is to'be understood that the capacitor 14 is preferably made sufficiently large to integrate the pulses 52 so that the positive swing of the anode 34 continues for a period well in excess of a sync pulse width. Therefore, the actual conduction period in the anode circuit of the vacuum tube 32 will be substantially a sole function ofthe duration of the sync pulse 16s, for intervals between vertical sync pulses, while being rendered a function of the duration of the actual integrated timing pulse applied to the anode 34 during the vertical sync pulse intervals. This latter effect will be slightly altered, of

course, by merit of the serrations in the present `the arrow H4 through resistors 16 and 11 will produce a negative potential with respect to ground at the anode 34. The magnitude of the negative `potential will, of course, be directly proportional to the received signal strength and may be, after suitable filtering, used directly as an automatic gain control potential. For example, a decrease inV the received signal strength will lower the instantaneous positive level of the grid 40 during the sync interval and will, therefore, reduce the current flow through the resistors 16 and 11 during the timing or keying pulses 62. This will render the anode 34 less negative or more positive. This positive trend of the anode will be -conveyed through the filter circuit 1li-18 and filter circuit 80-82 to the respective automatic gain control terminals 28 and 3E) to increase the gain of the receiver and thereby compensate for the reduction in received signal strength.

at the automatic gain control terminal 28. TheV bias clamp tube 84 as above-described insures that the gain control bias applied to the R. F. stages shall never swing positive on low level signals.

. I It is important to'lnotice that in the practice of the present invention, the connection of theA insertion grid 54 of the vacuum tube 32 to the anode 34 thereof is not necessary, although sometimes highly desirable. By means of this connection, it is possible to render the amplitude of separated sync more independent of the actual value of automatic gain control anode-cathode current in the discharge tube. However, by connecting the insertion grid 54 directly to the screen grids 42 and 43 of the vacuum tube 32,. it is possible to realize a slightly higher gain in the sync separation action. It is found important -to note that, although it is preferable to provide a certain amount of integration or broadening of the keying pulse 62` applied to the anode 34, that suchintegration is not entirely necessary.v In practice, it is desirable since it reduces the importance `of the timing of the yback pulse 62 being" in exact occurrence with the received sync pulses I6b.- It can bev seen that were integration noise immunity ofthe developed automatic gain control potential so that in practice, a compromise must be drawn between these two considerations.

Another embodiment of the present invention is shown in Figure 2. This embodiment is substantially Athe same as shown in Figure 1 with the exception of a few novel techniques which, in some instances, act to improve the waveform of the separated sync. For purposes of simplicity, circuit elements in Figure 2 corresponding to those in Figure 1 have been given like numerical designations. It will be noticed, however, in Figure 2 that the insertion grid 54 instead of being connected with the anode 34 is connected with the screen electrodes 42 and 44 through resistor H6. The insertion grid is then supplied with the output of the sync amplifier through capacitor H8. This produces a certain amount of regeneration which acts to sharpen or steepen the wave front of the separated synchronizing pulses. If desired, even more feedback may be applied by the resistor |20 connected between the anode ofthe sync amplifier 90 and the signal grid 40 of the vacuum tube 32. Care must be exercised in the use of the regeneration to maintain sufficient stability so that shock excitation of the circuit will not produce sustained or damped oscillation. However, by properly balancing the degree of feedback, the sync separation action may be improved as well as improving the effective automatic' gain control circuit gain and, thereby its ability to maintain the demodulated signal levelV at a constant amplitude.

Itis manifest that, although the regenerative feedback of the sync amplifier and automatic gain control circuit shown herein has been applied to a specific circuit configuration, its advantages and operation are in no way limited thereto. Controlled regeneration ofV this character will, of course, find use in other sync separating circuits as well as alternative forms of automatic gain control circuits.

As pointed out hereinbefore, gate type of sync clipping circuits to which the present invention may, in some respects, be considered similar, are particularly vulnerable to momentary increases in applied signal strength. For instance in the embodiments of Figure 1 and Figure 2, should the incoming signal, as intercepted by the antenna I2, increase to a very large value, the amplitude of the demodulated signal i6 applied to the vacuum tube 32 would also increase greatly. Since the bias between the grid 4U and cathode 36 would not instantaneously change, it will appear that the clipped output of the vacuum tube 32, appearing at the upper end of resistor 50, may well contain blanking pedestal as well as video information. This might cause the deflection generators 94 and 96 t-o falsely synchronize on components of the composite signal other than the synchro- Ynizing component lb. l

However, in further accordance with the present invention, mis-synchronization of the deflection circuit due to this effect is obviated by including the heretofore described resistor V48 and-capacitor 521in thevoltage supply circuit 9 pulses, means for applying said keying pulses to said load vpath for producing anode-cathode conduction therethrough, time constant means connected in said load path for developing a gain control potential in accordance with average current flow therethrough, connections from said receiver to said discharge tube first control electrode for applying demodulated composite signal thereto, means including a series load impedance for statically biasing said second conf trol electrode positively With respect to said cathode, means for dynamically biasing said rst control electrode suniciently negative relative to said receiver, means for coupling said second control electrode "load impedance voltage variations to said deflection generator for timing thereof, and connections applying said time constant gain kcontrol .potential to said gain control means for automatic gain control of said receiver.

5. In a television receiving apparatus adapted y to receive and demodulate a composite signal having a video component, a synchronizing pulse component and blanking component, said synchronizing pulses occurring during said blanking intervals and designated by a constant peak per- "centage of radio carrier modulation in excess of pulse extending in a positive direction to said rst control electrode With such amplitude to cause electron now only during peaks of synchronizing pulses, positively polarized load means connected with said second control electrode for developing clipped synchronizing signals, a discharge tube anode-cathode circuit excited by said keying pulses for developing a control potential representative of received signal strength, means for applying said control potential for automatic gain control of said receiver, a deflection generator for said television receiver, and means for applying said clipped synchronizing signals to said deflection generator for timing control thereof, said clipped synchronizing signals applying means including at least a second electron discharge tube having -a control electrode cathode and anode, a connection from said first discharge tube second control electrode to said second discharge tube control electrode, an anode-cathode load circuit for said second discharge tube, and means for -coupling the output of said second tube load circuit to said first discharge tube rst control electrode to obtain regenerative peaking of the clipped synchronizing signal information.

6. In a television receiving apparatus adapted to receive and demodulate a composite signal having a video component, a synchronizing pulse component and blanking Component, said synchronizing pulses occurring during said blanking intervals and designated by a constant peak percentage of radio carrier modulation in excess of said blanking component, a combination synchronizing pulse clipper and automatic gain control system comprising in combination, an electron discharge tube of the pentagrid converter variety having a signal grid, an insertion grid, a screen grid structureon either side of said insertion grid, a suppressor grid, an anode and a cathode, a source of keying pulses synchronously related to lthe received synchronizing pulses, said keying pulses being of greater duration than said synchronizing: pulses, means for statically biasing said signal grid to electron current cutoi in said tube, means for applying demodulated compositesignal with the synchronizing pulse extending in a positive direction to said signal grid with such amplitude to cause electron flow only during peaks of synchronizing pulses, positively polarized load means connected with said screen grid structureA for developing clipped synchronizing signals, a discharge tube anode-cathode 4circuit excited by said keying pulses for developing a control potential representative of received signal strength, means for applying said control potential for automatic gain control of said receiver, a deiiection rgenerator for said television receiver, and means for applying said clipped synchronizing signals to said deflection generator for timing control thereof, said clipped synchronizing signals applying means including at least a second electron discharge tube having a grid, cathode and anode,

an anode-cathode circuit for said second dis- 'charge tube, and means for coupling the load circuit of said second discharge tube to said first tube insertion grid. f

7. Apparatus according to claim 6 wherein there is further provided a resistor between said screen grid structure and said insertion grid of said first-named discharge tube.

8. In a television receiving apparatus adapted to receive and demodulate a composite signal having a video component, a synchronizing pulse component and blanking component, said synchronizing pulses occurring during said blanking intervals and designated by a constant peak percentage of radio carrier modulation in excess of said blanking component, a combination synchronizing pulse clipper and automatic vgain control system comprising in combination, gain control means for controlling the gain of said receiver in accordance with a control potential, a pentagrid converter type electron discharge tube having at least a signal grid, insertion grid, screen grid, cathode and anode, a statically nonconductive load path Aconnected between the anode and cathode of said discharge tube,l a source of keying pulses synchronously related to the received synchronizing pulses, means for applying said keying pulses to said load path for producing anode-cathode conduction therethrough, time constant means Aconnected in said load path for developing a gain control potential in accordance with average current'flow therethrough, connections from said receiver to said discharge tube signal grid for applying demodulated composite signal thereto, means including a series load impedance for statically biasing said screen grid positively with respect to said cathode, means for biasing said signal electrode suiliciently negative relative to said cathode to permit current now through said screen elecerator for said television receiver, means for coupling said screen electrode voltage variations to'said defiection'generator for timing thereof, and connections applying said timeconstant gain control potential to said vgain control means for automatic gain control ofsaid receiver.

9. Apparatus according-to claim 8 wherein said insertion grid is directly connected to said'screen grid.

10. Apparatus according lto claim 8 wherein said insertion grid is directly connected to said discharge tube anode.

11. Apparatus according to claim 8 wherein said means for coupling said screen electrode voltage variations to said deection generator embracesea secondelectron'discharge tube having a grid, cathode and anode, with said grid being coupled to'said first-named discharge tube screen electrode, an anode-cathode load circuit for said second discharge tube, and means for coupling the load circuit offsaid second discharge tube to said rst tube insertion grid.

12. Apparatus according to claim 8 wherein said means for coupling said screen electrode voltage variations to said deflection generator embraces a, second electron discharge tube having a'grid, cathode and anode, with said grid being coupled to said rst-named discharge tube screen electrode, an anode-cathode load circuit for said second discharge tube, and a resistance connected from the-anode of said second discharge tube to the-signal grid of said rst discharge tube.

BENJAMIN S. VILKOMERSON.

CLYDE W. HOYT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,197,900 Schlesinger Apr. 23, 1940 2,231,792 Bingley Feb. 11, 1941 2,289,291 Moore July 7, 1942 2,307,218 Hardwick Jan. 5, 1943 2,356,141 Applegarth Aug. 22, 1944 2,538,519 Holst Jan. 16, 1951 2,559,038 Bass July 3, 1951 2,566,763 Fyler Sept. 4, 1951 FOREIGN PATENTS Number Country Date Italy Dec. 16, 1939 

